This application incorporates by reference Taiwanese application Serial No. 089126649, filed Dec. 13, 2000.
1. Field of the Invention
The present invention generally relates to a liquid crystal display (LCD) with a wide viewing angle, and more particularly to a LCD having a regulating device for aligning liquid crystal molecules with a negative dielectric constant.
2. Description of the Related Art
Benefited from the advantages of the thinness, lightness and low radiation properties, LCDs (Liquid Crystal Displays) have been widely used in the world. However, the contrast ratio will be decreased as the user sees the display from a larger viewing angle, which causes the viewing angle to be within a very limited range. Therefore, how to increase the viewing angle and improve the property of the LCD has been the critical lesson for the researchers.
Referring to FIG. 1, it is a sectional view showing the structure of a conventional LCD. The conventional LCD has an upper plate 102 and a lower plate 104. An upper analyzer film 106 is positioned above the upper plate 102 and a lower polarizer film 108 is positioned under the lower plate 104. The polarization direction of the upper analyzer film 106 is perpendicular to that of the lower polarizer film 108. Moreover, the upper plate 102 includes a glass substrate 110, a color filter 112, a transparent electrode 114, and an alignment film 116. The lower plate 104 includes a glass substrate 120, a transparent electrode 122, and an alignment film 124. The liquid crystal layer 118, including a plurality of liquid crystal molecules 118A, is sandwiched between the upper plate 102 and lower plate 104. After a rubbing process, these liquid crystal molecules are aligned on the alignment films 116 or 124.
When a voltage Va is supplied between the transparent electrode 114 and transparent electrode 122, the alignment of the liquid crystal molecules in the liquid crystal layer 118 will be changed according to the amount of the voltage Va. The direction of light will varied because of different alignment directions of the liquid crystal molecules 118A. Under this condition, the transmittance of the light will be changed according to the state of the liquid crystal molecules 118A in the liquid crystal layer 118. Therefore, the various brightness of the LCD, such as white, dark and gray scale, can be controlled by the various voltage applied between the transparent electrodes 114, 112.
In order to get a wide viewing angle, a LCD using an OCB mode (Optically Compensated Bend mode) has been provided. Referring to FIG. 2, it shows the relation between the applied voltage Va and the transmittance of the LCD with the OCB mode. As the applied voltage Va is 0, the transmittance is T1. If the applied voltage Va equals to the threshold voltage Vc, the transmittance will be the maximum value, T2. When the applied voltage Va is V1, the transmittance is the minimum value 0.
In the OCB mode, the dielectric constant difference of the liquid crystal used in the LCD is positive, and the liquid crystal molecules are in a horizontal alignment on the alignment films 116, 124. The direction of the upper analyzer film 106 is perpendicular to that of the lower polarizer film 108.
Referring to FIG. 3Axcx9c3C, the arrangement of the liquid crystal molecules in the liquid crystal layer 1118 are varied by different voltages Va applied to the LCD with the OCB mode. In FIG. 3A, 3B, 3C, the applied voltage Va is 0, Vc, and V1, respectively. The liquid crystal layer 118 comprises a first liquid crystal layer A, a second liquid crystal layer B and a third liquid crystal layer C. The first liquid crystal layer A is contact with the alignment film 116, the second liquid crystal layer B is contact with the alignment film 124, and the third liquid crystal layer C is sandwiched between the first liquid crystal layer A and the second liquid crystal layer B.
In FIG. 3A, the applied voltage Va is zero at the initial state, the angle between the first liquid crystal layer A and the alignment film 116 or the second liquid crystal layer B and the alignment film 124 is very small, about 3 to 8 degree. The liquid crystal molecules in the third liquid crystal layer C turn to parallel to the alignment films 116, 124. This is in a xe2x80x9csplay alignmentxe2x80x9d state. At this time, angles are formed between the liquid crystal molecules and both of the upper and lower analyzers 106, 108, so lights can pass the upper analyzer film 106 through the lower polarizer film 108. The LCD is in a white state.
In FIG. 3B, as the applied voltage Va is Vc, the angle between the first liquid crystal layer A and the alignment film 116 or between the second liquid crystal layer B and the alignment film 124 is increased. The liquid crystal molecules in the third liquid crystal layer C are almost perpendicular to the alignment films 116, 124. This is in a xe2x80x9cbend alignmentxe2x80x9d state. At this time, the LCD is in the white state and has a maximum brightness.
In FIG. 3C, as the applied voltage Va is V1, the angle between the first liquid crystal layer A and the alignment film 116 or between the second liquid crystal layer B and the alignment film 124 is increased to a maximum, about 80 degrees or more. The liquid crystal molecules in the third liquid crystal layer C are substantially perpendicular to the alignment films 116, 124. In other words, the aligned direction of all liquid crystal molecules are almost perpendicular to the upper analyzer 106 and lower polarizer 108, so lights can""t pass through these polarizers. At this time, the LCD is in a dark state.
The OCB mode works when the applied voltage Va between the upper and lower plate 102, 104 is in a range between the threshold voltage Vc and V1. The various brightness and the gray scale of the LCD are achieved by various voltage Va. In the OCB mode, the liquid crystal molecules can be arranged well, the liquid crystal molecules rotate in the same direction, and the friction between the liquid crystal molecules is reduced. Therefore, the LCD with OCB mode has the advantages of fast response and the wide viewing angle. However, it is unstable when the applied voltage is in the range between zero and the threshold voltage Vc. In this unstable stage, the LCD with the OCB mode is non-operated. In order to operate the LCD, the applied voltage should be higher than the threshold voltage Vc. It takes a time to increase the applied voltage Va from zero to the threshold voltage Vc. In other words, the response time of the LCD with OCB mode is slow and the applied voltage Va is high.
From the above description, the object of the present invention is to provide a LCD having a wide viewing angle. The LCD is in the dark state without applied voltage and in a white state with the applied voltage, such that the contrast ratio of the LCD is increased. A plurality of domains are defined by opposite regulating devices on the upper and lower substrates to achieve the advantages of the wide viewing angle, high contrast ratio and high response speed.
According to the present invention, a liquid crystal display having a wide viewing angle includes a first substrate, a first electrode, a first regulating device, a second substrate, a pixel electrode, a second regulating device and a liquid crystal layer. The first substrate includes a first surface thereon. The first electrode and the first regulating device having a first inclined plane are formed on the first surface. The second substrate has a second surface, and the first surface of the first substrate is opposed to the second surface of the second substrate. The pixel electrode and the second regulating device with a second inclined plane are formed on the second substrate. The first regulating device is opposed to the second regulating device. The liquid crystal layer is positioned between the first substrate and the second substrate. The liquid crystal layer includes a plurality of liquid crystal molecules with a negative anisotropic dielectric constant. The liquid crystal layer comprises a first liquid crystal molecule in the proximity of the first regulating device, a second liquid crystal molecule in the proximity of the second regulating device, and a third liquid crystal molecule between the first and second liquid crystal molecules.
While no driving voltage is applied between the first electrode and the pixel electrode, (a) the first liquid crystal molecule is perpendicular to the first inclined surface, a first angle is formed between the first substrate and the first liquid crystal molecule, and the first angle is an acute angle, (b) the second liquid crystal molecule is perpendicular to the second inclined surface, a second angle is formed between the first substrate and the second liquid crystal molecule, and the second angle is an obtuse angle, (c) the third liquid crystal molecule is perpendicular to the first substrate, such that the angle between the first and third liquid crystal molecules and the angle between the third and second liquid crystal molecules are both obtuse angles.
While a driving voltage applied between the first electrode and the pixel electrode, an oblique electric field is formed between the first substrate and the second substrate because of the first regulating device and the second regulating device. The first liquid crystal molecule rotates along a first direction, the second liquid crystal molecule rotates along a second direction, and the first direction is contrary to the second direction. Therefore, all liquid crystal molecules are almost parallel to the first substrate because of the oblique electric field.
In addition, the LCD is in a dark state while no driving voltage is applied between the first electrode and the pixel electrode. The LCD will be in a white state when a driving voltage is applied between the first electrode and the pixel electrode.
Moreover, a plurality of pixel areas are defined between the upper plate and lower plate. Each pixel area is divided to many domains by the first regulating device and the second regulating device.
These advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.